When hair is wet, it's fragile and extremely susceptible to damage. Hair can easily be ripped, torn and split while it is wet.

Why does this happen? It's because of Hydrogen Bonds. Here's the science behind it:

When the hydrogen bonds in the hair cortex become wet, the water breaks down the natural bonds that hold onto the keratin proteins in the hair cortex. The hydrogen bonds form again when hair is dry but until then, they are easily damaged.